1. Field of the Invention
This invention relates to a method and apparatus for coating paperboard and, more specifically, a method and apparatus for producing coated paperboard products with a low moisture vapor transmission rate (MVTR), excellent glueability, printability, and recyclability.
2. Description of the Related Art
Corrugated board is a converted or remanufactured paper product. It is a layered structure that is usually die cut to form corrugated containers. It consists of a fluted corrugated medium sandwiched between sheets of linerboard. The simplest three-ply structure is known as "double face." As recently as 1990, much of the linerboard was made entirely from virgin, long-fibred, softwood, kraft pulp. Today, however, these board grades contain sizeable portions of recycled old corrugated containers (OCC) and many are made from 100% OCC.
Around the country, and even around the world, landfill space for waste disposal is rapidly reaching capacity. By the year 2000, paper and paperboard products are projected to represent 40.9 percent of the municipal solid waste stream and may climb to nearly 42 percent by 2010. New governmental regulations and the public's increasing concern for the environment have created pressure to remove these materials from the solid waste stream. The most widely utilized method of reducing paper waste is recycling.
OCC has a history of efficient recycling use. Even before the era of government mandates and self-imposed industry goals, almost 50% of OCC was recycled in North America. Today's recovery rate is about 62%. It is expected that a level of 70% will be achieved by the year 2000. Today, most of this recycled material goes directly from retail chain stores to mills based on long-term contracts. The rest comes from municipal curbside collection and wastepaper dealers. Some OCC is used in the production of boxboard, and some is even bleached and used in the production of fine paper, but most OCC is used again to produce corrugating medium and linerboard.
"Repulping" refers to any mechanical action that disperses dry or compacted pulp fibers into a water slush, slurry or suspension. The action can be just sufficient to enable the slurry to be pumped, or it can be adequate to totally separate and disperse all the fibers. In a typical recycling process, bales of OCC are fed into a repulper where the material is disintegrated and the gross contaminants are removed. The resulting stock is pumped through pressure screens and cyclonic cleaners to remove oversized materials and foreign matter. Plastics, styrofoam or other lightweight contaminants are removed by reverse cleaners. The glue, staples, wax, and tapes originally used to assemble the corrugated box must be removed.
Untreated OCC usually creates no problems for recycling. However, paperboard is often treated or coated to enhance its performance and these coatings render the paper unrecyclable. For example, corrugated paperboard is often treated with a curtain coating, wax impregnation, lamination, sizing, or a water-based coating to reduce abrasiveness and to provide for oil and moisture resistance. While coatings such as wax enhance the moisture resistant properties of the paperboard, the wax coating process is expensive and often renders the paperboard unrecyclable. Therefore, a need exists in the art for a coating system that enhances the usability of corrugated boxes while still allowing the containers to be recycled.
While various coatings have been introduced in the market to provide a recyclable coating, they suffer from limitations such as printability and gluablity. Some repulpable coatings cannot be printed over due to a high degree of water resistance or the release characteristics of the coating. While some repulpable coatings may be printable, they require special inks. More detrimentally, many currently available repulpable paperboard coatings are not compatible with all glues. For example, some coatings are compatible only with hot-melt adhesives only, while others, by nature of their heat-resistant and oil-resistant properties, repel the oil-based hot-melt adhesives. Therefore, a need exists in the art for a coating system that is compatible with a broad range of printing and gluing systems.
Moisture vapor transfer rate (MVTR) is a scientific measurement used to describe a product's ability to allow moisture vapor to pass through it, over a specific time period, at a controlled temperature and at a designated atmospheric pressure. Products as diverse as frozen meat patties and fine papers are extremely sensitive to moisture gain or loss. In the case of meat patties, excessive loss of moisture while product is being held in a freezer results in "freezer burn." Freezer burn is unsightly and adversely affects the taste of the cooked meat. For fine papers, excessive moisture gain results in limp, hard-to-process sheets. Traditional solutions generally involve plastic film, either as a laminate with the paper or as a bag around the product. Both solutions are expensive or incur added labor costs, and greatly reduce or eliminate the recyclability of the shipping container. Therefore, there exists a need in the art for coatings that can provide the high moisture resistance needed without compromising the recyclability of the container.
The MVTR of a container is dependent not only upon the coating on the corrugated paperboard, but also the method by which that coating is applied. Traditional methods of coating application, such as a rod coater or a blade coater, may result in variations in coating thickness that will cause variations in the MVTR of the coating. The typical solution to this problem has been to merely increase the amount of coating applied to the paperboard. This solution can be expensive and does not result in a consistently coated product both linearly and across the web. Therefore, a need exists in the art for a process for applying a uniform coating to paperboard with very little variation in MVTR.